This invention relates generally to a system for unwinding and rewinding a ribbon-like rewind medium and more particularly to such a system wherein the tension in the rewind medium is maintained substantially constant by intermittently driving a take-up spool by the actuation and deactuation of a rewind motor.
When a rewind medium is fed at a first rate to a take-up spool which is operated at a different or intermittent rate, there are, of course, repeated variations in the ribbon tension. These variations may result in tangling, stretching or breaking the rewind medium. When the rewind medium has a magnetic oxide coating or an ink coating or the like, such as are used in the tape recording or printing arts, excess stretching of the rewind media can cause fragments of the oxide coating or ink coating to break off thereby rendering corresponding portions of the rewind medium useless. Prior art solutions to these problems generally provide apparatus designed to maintain the torque developed by the take-up spool drive motor within specific narrow limits that will not damage the rewind medium.
Such prior art solutions have utilized either complex circuitry to provide the rapidly responsive takeup spool motor control that is required to restrict the torque within these narrow limits, or more modest circuitry which results in a higher cyclic rate of take-up spool operation.
The use of complex electrical circuitry greatly increases the cost of the system and makes maintenance and diagnostic functions more difficult. The use of the more modes circuitry with the high cyclic rate of operation usually results in shorter component life.
A few of the systems which employ modest circuitry and have a higher cyclic rate of take-up spool operation use a reed switch to control the operation of the rewind motor. The reed switch is positioned in proximity to a magnetic member and a shunt plate is positioned between the reed switch and the magnetic member. So long as the shunt plate is positioned between the reed switch and the magnetic member, the switch remains deactivated and the rewind motor remains off. A few of these systems employ a shunt-positioning member which is directly responsive to the tension in the rewind medium for controlling the positioning of the shunt plate such that a lessening in tension causes the shunt plate to withdraw from a shunt position disposed between the reed switch and the magnetic member thereby causing the reed switch to become actuated and the rewind motor to operate to take up the slack. When the tension is increased, the tension-responsive shunt-positioning means will reinsert the shunt plate between the reed switch and the magnetic member thereby deactivating the switch and shutting off the rewind motor.
The problem with such systems is that the tension-responsive members tend to respond to slight reductions or variations in the tension of the rewind media thereby causing the reed switch to be continually turning off and on so as to cause the rewind motor to operate in a large number of short spurts. This in turn tends to cause "cinching" or excessive tightness in the rewind media which, in addition to wasting energy and causing excessive wear on the motor and associated rewind mechanisms, has the effect in many cases of causing the take-up reel to buckle or bind.